$\begingroup$Are you by any chance running on an intel embedded GPU? I've also seen this happening on an intel laptop with Intel GM965/GM960 integrated graphics on linux. Perhaps a hardware or driver issue? It shouldn't affect the game in any way, so I've just ignored it (or used the external player)$\endgroup$
– gandalf3♦Mar 3 '18 at 5:19

1

$\begingroup$I can confirm, this is a typical Intel GPU/Driver issue.I had that at a couple of different laptops with similar effects.$\endgroup$
– MonsterMar 6 '18 at 5:28

$\begingroup$I observed this as on a Laptop with Intel HD Graphics 520 as well.$\endgroup$
– AeroLynxMar 10 '18 at 12:38